Rajan Wilcox
Aerodynamic cars are designed to slip smoothly through the air, reducing drag and improving fuel efficiency. The study of airflow around a car is known as aerodynamics, and it plays a crucial role in a vehicle's performance, speed, and fuel economy. The smoother the airflow, the lower the drag, and the less fuel a car burns at a specific speed. This principle is measured by the drag coefficient (Cd), which indicates how easily a vehicle moves through the air. Automakers are increasingly focusing on aerodynamic design to improve fuel economy, and small changes in vehicle design can have a significant impact on fuel efficiency.
| Characteristics | Values |
|---|---|
| Drag Coefficient (Cd) | The lower the Cd, the more fuel-efficient the vehicle. |
| Frontal Area | Width x Height of the front of the vehicle. Smaller frontal area means better fuel efficiency. |
| Drag Area | Total of Cd multiplied by the frontal area. Smaller vehicles have lower drag areas and better fuel efficiency. |
| Downforce | Improved downforce leads to better tire grip, handling, and speed. |
| Lift | Aerodynamic efficiency is a factor in reducing costs and fuel consumption. |
| Yawing Moment | The effect of crosswinds on the vehicle's steering. |
| Noise | Increased noise is a sign of increased drag and reduced aerodynamics. |
| Air Density | Not a variable that can be controlled. |
| Speed | Aerodynamic drag increases with the square of the speed. |
| Friction and Rolling Resistance | Responsible for most fuel burned at urban speeds. |
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What You'll Learn
Coefficient of drag (Cd)
The coefficient of drag, also known as Cd, is a figure published by carmakers to indicate a vehicle's aerodynamic efficiency. It essentially signifies how easily a vehicle moves through the air, with a lower Cd indicating greater fuel efficiency. For instance, the Nissan Leaf EV has a Cd of 0.29, while the Chevrolet Volt plug-in hybrid has a Cd of 0.28, making them both more efficient than most cars today.
The impact of Cd on fuel efficiency is significant. According to GM's Schenkel, a change in the drag coefficient of 0.01 can improve a full-size truck's fuel economy by about 0.1 mpg and a car's fuel economy by about 0.2 mpg. This improvement is even more pronounced in vehicles with higher horsepower powertrains. For example, the 2014 Ferrari Enzo, with its 920-horsepower powertrain, achieved a 30% improvement in fuel efficiency due to weight reduction and aerodynamic efficiency.
Several factors contribute to aerodynamic drag, including skin friction, turbulence, and pressure differences. Carmakers employ various methods to minimise these factors and improve Cd, such as redirecting airflow over and around a vehicle's surfaces using separators or squared corners at bumper and fender junctions. Additionally, sealing gaps in the undercarriage and using smoother wheels can help reduce drag.
While Cd is a crucial factor in fuel efficiency, it is not the only consideration. Downforce, lift, and yawing moment also play a role in a vehicle's overall aerodynamic performance. Nevertheless, by focusing on improving Cd, automakers can enhance fuel economy without making radical changes to a vehicle's design or incurring high costs associated with weight reduction or engine efficiency improvements.
In conclusion, the coefficient of drag (Cd) is a critical factor in determining a vehicle's fuel efficiency. By improving Cd through aerodynamic design, automakers can create vehicles that are not only more fuel-efficient but also environmentally and financially beneficial.
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Smoother airflow
The smoother the airflow around a car, the lower the drag, and the less fuel the car will burn at a specific speed. This is why carmakers are so keen to develop their vehicles' aerodynamic efficiency—it means they can achieve better gas mileage figures and sell more cars. It is also much cheaper for carmakers to improve efficiency with aerodynamics than by expensive weight-saving methods or costly engine technologies.
The drag coefficient (Cd) is a number that signifies how easily a vehicle moves through the air. The lower the drag coefficient, the more fuel-efficient the vehicle will be. The most aerodynamically efficient production car in modern history is the EV1 electric car built by General Motors, which boasted a 0.19 Cd. Most cars these days have a Cd that ranges between 0.30 and 0.35, while most trucks and SUVs have a Cd that ranges between 0.35 and 0.45.
Automakers have found ways to make their vehicles slip more smoothly through the air. For example, the Nissan Leaf EV uses a separator that redirects airflow for smoother passage over and around a vehicle's surfaces. The odd-looking housing directs airflow over and under the outside mirrors, minimizing the flat surfaces that increase wind resistance and create aerodynamic drag. Even small changes can have a big impact on decreasing drag. For example, squared corners at bumper and fender junctions smooth airflow, while panels seal the gaps in the undercarriage that create drag.
Additionally, adding performance parts like a rear diffuser or a rear spoiler can help with reduced lift and improved downforce, as well as fuel efficiency during everyday driving.
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Downforce and lift
Aerodynamic components like splitters, dive planes, and rear wings are designed to increase downforce. These elements manipulate the airflow around the vehicle to create a downward push, improving traction and control. While these additions do increase fuel efficiency, they also increase drag, which is why finding the right balance is crucial.
Lift, on the other hand, is the upward force acting on a vehicle as it moves through the air. It is a result of air flowing over and under the car's surfaces, creating a vacuum effect above and below. This force counteracts the weight of the vehicle, reducing the tyres' grip on the road.
To minimise lift, car designers employ various strategies. One common approach is to use a separator, which redirects airflow to smoothen its passage over and around the vehicle, reducing the flat surfaces that increase wind resistance and lift. Additionally, squared corners at bumper and fender junctions help to smooth airflow, while panels seal gaps in the undercarriage that can create lift.
The interplay between downforce and lift is a delicate balance. While downforce is desirable for improved handling and stability, excessive lift can be detrimental, causing reduced tyre grip and unstable handling. As a result, automotive engineers strive to optimise these forces to achieve the desired level of downforce without incurring excessive lift, ultimately enhancing fuel efficiency.
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Yawing moment
The yawing moment is a crucial factor in vehicle design and fuel efficiency. It refers to the impact of crosswinds on a vehicle's steering and stability. When a car experiences a strong side wind, the yawing moment can cause the vehicle to veer off its intended path, requiring the driver to correct their course. This phenomenon is essential to understand and mitigate to ensure safe and efficient driving, especially when travelling at high speeds or in challenging weather conditions.
Aerodynamic design plays a significant role in minimising the effects of the yawing moment. By optimising the airflow around the vehicle, designers can reduce the impact of crosswinds and improve stability. This is achieved through careful shaping and streamlining of the car's body, including the use of smooth surfaces, rounded corners, and sealed gaps, all contributing to a more stable and fuel-efficient driving experience.
Additionally, certain performance parts can be added to enhance the vehicle's resistance to the yawing moment. For instance, front-end components such as splitters and dive planes can be utilised to balance out the effects of an aggressive rear wing, improving stability and fuel efficiency. A well-designed rear diffuser can also play a crucial role in reducing aerodynamic drag and enhancing the vehicle's overall performance.
The yawing moment is not just a safety consideration but also a financial one. By improving a vehicle's resistance to the effects of crosswinds, automakers can enhance fuel efficiency and reduce costs for consumers. This focus on aerodynamics allows for better gas mileage, benefiting both the environment and the driver's wallet.
In conclusion, the yawing moment is a critical aspect of vehicle design and performance. By optimising aerodynamic features and utilising appropriate performance parts, automakers can minimise the impact of crosswinds, improve stability, and enhance fuel efficiency. This not only ensures a safer driving experience but also contributes to cost savings and environmental sustainability.
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Aerodynamic parts
- Rear Diffuser: A rear diffuser helps reduce aerodynamic drag and improve downforce, leading to better fuel efficiency. The Subaru BRZ, for example, features a rear diffuser that enhances its aerodynamic performance.
- Front-end Components: Front-end parts like splitters and dive planes are essential to balance the effects of an aggressive rear wing. These components improve airflow, downforce, and overall vehicle handling, contributing to better fuel efficiency.
- Separators: Separators are used to redirect airflow, ensuring smoother passage over and around a vehicle's surfaces. The Nissan Leaf EV, for example, employs separators in its headlamp assembly to minimize flat surfaces, reducing wind resistance and aerodynamic drag.
- Smooth Wheels: Opting for smoother wheels, similar to those on land speed record-breaking vehicles, can enhance aerodynamics. Flush discs, in particular, improve airflow and reduce drag.
- Rear Spoiler: Adding a rear spoiler can, in certain cases, improve aerodynamics and reduce drag.
- Aerodynamic Body Design: The shape and design of a car's body significantly impact its aerodynamics. A vehicle with a low, rounded nose and a squared-off tail, like the Ferrari Enzo, benefits from improved airflow and reduced drag.
- Waxing and Polishing: While there is no definitive evidence, some experts suggest that waxing and polishing your car can reduce skin friction, leading to improved aerodynamics and, consequently, better fuel efficiency.
- Air Dam and Diffuser: Ensuring that your air dam and diffuser are in good condition is crucial for maintaining aerodynamic efficiency. A broken or damaged air dam or diffuser can increase drag and negatively impact fuel economy.
These aerodynamic parts and design considerations collectively contribute to reducing aerodynamic drag, improving airflow, and enhancing fuel efficiency in vehicles.
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Frequently asked questions
The coefficient of drag, or Cd, is the primary factor in determining a car's fuel efficiency. This number signifies how easily a vehicle moves through the air. The lower the drag coefficient, the more fuel-efficient the vehicle.
Aerodynamics is the study of airflow around a car. The smoother the airflow, the lower the drag, and the less fuel burned at a specific speed. Aerodynamic efficiency is a big factor in reducing costs, both for the individual and the environment.
Car manufacturers use wind tunnels and teams of aerodynamic experts to improve fuel efficiency. They focus on unseen details like airflow redirection, smoother wheels, and squared corners at bumper and fender junctions to minimize flat surfaces and smooth airflow.
The Nissan Leaf EV, Chevrolet Volt plug-in hybrid, and Toyota Prius hybrid are examples of cars with excellent fuel efficiency due to their aerodynamic design. The Nissan Leaf EV, for example, has a Cd of 0.29, contributing to its impressive fuel economy.
